Various processes are known for forming a layer of plastic material by casting a liquid on a substrate. For instance, by forming a film drawn from a meniscus. In this case, a meniscus of material is formed and maintained by surface tension between the slot of the distributor of liquid material and the substrate. By subsequently moving the substrate, the liquid is continuously extracted from the meniscus while the meniscus is continuously resupplied by the liquid distributor. This process requires a relationship between the interfacial tension between the substrate and the liquid, on one hand, and the surface tension of the liquid on the other hand, and it cannot be used to deposit liquids which do not wet the support. Moreover, with this process, it is particularly difficult to obtain a deposit of uniform thickness. Further, defects can be caused by minute imperfections in the distribution slot. These factors militate against consistently forming a layer with good enough optical quality for use in laminated glazing, for example.
Another known process involves depositing material on a substrate by extrusion of the fluid material. In this case, all of the cast material is directly entrained by the mobile substrate for layer formation. The flow is laminar. It is difficult to obtain thin layers, i.e., on the order of a few tenths of a millimeter or less in thickness, with this technique.
Another process for depositing material on a mobile substrate for layer formation is known. In this process, the material forms a bead above the substrate which spreads in a direction opposite to the direction of advancement of the substrate. This process permits formation of a thin, uniform layer, regardless of the wetting properties of the cast fluid material. However, the bead usually contains dead zones in which the material stagnates. This is particularly troublesome when forming a transparent layer by depositing a mixture of components which react within each other during the depositing process. In this case, the material which is present in the dead zones of the bead is older than the material deposited directly on the substrate. Vibration of the substrate can cause a disturbance in the bead and its stagnant zones and, consequently, possible entrainment by the substrate of material from the stagnant zones. This results in the formation of a heterogeneous film having portions of very different ages, that are in more or less advanced stages of polymerization. Optical defects, such as streaks, are then formed and the film cannot be used in laminated glazing.